1. Field of the Invention
The present invention relates to a glass fiber mat. More particularly, it relates to a glass fiber mat useful as a reinforcing material of a fiber reinforced plastic material (FRP) and a method of preparation thereof.
2. Description of the Prior Art
It has been known to prepare a glass fiber mat by collecting a large number of glass fibers drawn from bushings while applying a size to said glass fibers and gathering the glass fibers into a strand by means of collecting device and bringing the strand into contact with the surface of a pulling wheel rotated at high speed to pull the strand at high speed and throw it so as to pile on a moving conveyor in a looped and a non-oriented state while applying a mat binder. Japanese Utility Model Publication Sho No. 43-234, for example, discloses a process for preparing the mat (referred to as the wheel pulling process) in detail.
A mat prepared by the wheel pulling process consists of a large number of continuous strands arranged in a looped and non-oriented state and only the contacting point of each strand is bonded with a binder such as synthetic resin. The mat is referred to as a continuous strand mat (simply referred to as CSM) and is widely used as the reinforcing material of FRP because it has an excellent property as follows.
The strands for CSM are bonded together in a looped and non-oriented state so that the CSM becomes bulky. Accordingly, the bulk density of the CSM can be smaller than that of a mat formed by chopped strands. When the CSM is used as a material of FRP, the breaking of the mat does not occur even in the deep drawing operation by the press molding.
The CSM has an excellent wet-through property of resin so that the processability is excellent because a uniform impregnation of the resin into the CSM can be attained when a liquid resin is applied to the CSM.
However, the CSM (used as a reinforcing material) is impregnated with resin and the resin is cured to provide FRP and if a resin having a large shrinkage rate is used, individual strands forming the CSM appears on the surface of FRP like a line during the curing of the resin; thus a smooth surface is not obtained.
A chopped strand mat (simply referred to as CM) is known as one of the glass fiber mats used as the reinforcing material of FRP.
CM is most commmonly used for FRP and is prepared from a cake as a starting material. The cake is usually prepared by collecting a large number of glass fibers into a strand while a size is applied to the glass fibers and winding the strand, giving a reciprocal traverse movement, on a tube made of paper which is inserted on a mandrel rotated at high speed. The cake is usually dried by heat to cure the size and then is fed to the following working place. A large number of cakes are employed and a strand is drawn from each cake simultaneously and is fed to a cutter to be cut at a predetermined length. The resulting chopped strands are piled on a moving conveyor and a mat binder is applied to the chopped strands. The chopped strands (simply referred to as CS) are heated to cure the mat binder. The length of CS is usually several cm. However, longer CS such as those with a length in the range of 10 to 15 cm are used if necessary.
CS can be formed by cutting a roving which is composed of a plurality of the strands.
CM is economical and is suitable for mass production and is widely used as a reinforcing material of FRP. When CM is used in the hand lay-up process, a good mold-conformability can be attained and the operation is easy. However, when a force is applied at a local portion of CM, that portion elongates to decrease the strength in the elongated portion. When a deep drawing operation is carried out, the deep drawn portion in the CM is easily broken disadvantageously.
The strand used in the CSM is continuously looped so that the CSM used as the reinforcing material of FRP is free from the decreasing of strength in any local portion due to the local elongation of the mat in the molding operation as with the use of CM. Thus, the use of CSM provides FRP having great strength. Further, CSM has a good mold-conformability and a breaking resistance even in the deep drawing operation. However, the CSM has the following disadvantages. The CSM is more expensive than the CM, the demand is small and products having various qualities can not easily be prepared because production equipment using the wheel pulling process has a direct connection with bushings resulting in a difficulty of the job change (change of kind of product).
Furthermore, there is following limitation in the process for preparing the CSM.
As described above, the CSM is prepared by collecting the glass fibers drawn from the bushings into the strand and piling the strand on the moving conveyor directly. The diameter of the glass fiber drawn from the bushing is apt to be smaller when the drawing speed is increased. In order to obtain a desirable diameter in the range of about several .mu.m to ten and several .mu.m, the drawing speed of the glass fiber should be 1000 m/min. to 2000 m/min. (in the wheel pulling process).
The moving speed of the conveyor is determined depending on the amount of the strand to be piled per unit area of the conveyor and it is usually in the range of about 5 to 20 m/min. because of a limitation of the structure of the conveyor.
Accordingly, the drawing speed of the glass fiber is greatly higher than the moving speed of the conveyor so that the strand is piled on the conveyor in the condition that the strand is continuously looped in a small circle and the loops are slightly shifted.
The cross section of strand wound as the cake has a ratio of a long radius to a short radius in the range of 10:1 to 2:1 so as to be a flat circle in cross-sectioned shape because the strand is tightly wound on the tube and successively wound thereon so that the individual strand are deformed. On the other hand, the strand produced by the wheel pulling process is merely in contact with the surface of the wheel and is thrown so that the strand is not deformed and keeps its circular cross sectioned shape which is resulted by the interfacial tension of the liquid size.
As described above, the CSM is composed of the strand having a circular cross section which is looped in a small radius of curvature wherein each loop is overwrapped with a slightly shifting state thereby providing a great repulsive force to a compression. It is, therefore, difficult to increase the content of glass strand per unit volume compared with CM. This tendency is greatly enhanced in the case using the hand lay-up process. (The content of glass strand per unit volume is an important factor in improving the properties of FRP such as tensil strength, bending strength etc. When the glass content is small, it is difficult to obtain FRP having large strength.)
The inventors of this application have further studied on the basis of the knowledge and the findings described above, in order to overcome the disadvantages of the conventional mat. As a result, the inventors have found that a better result can be obtained when a wound strand (the strand is preferably dried) is drawn to pile thinly and in a non-oriented state on a glass fiber mat prepared by the wheel pulling process, said wound strand being formed by collecting a large number of glass fibers drawn from bushings, while applying a size, into a strand and by winding the strand on the rotating mandrel.